Photoswitching processes

Figure 5.35 Schematic illustration of a photoswitch process in which the photoreaction, in this instance, photoisomerization, occurs at the SAM. This alters the ability of the monolayer species to recognize and interact with a redox-active solution-phase moiety...

Photochemical switches are also discussed in Special Topic 6.15 (Scheme 6.161) and Special Topic 6.19 (Scheme 6.207). Here we illustrate the photoswitching process, which can control the geometry of biomolecules.1101 When an azobenzene-derived cross-linker in the DNA-recognition helix of the transcriptional activator MyoD is irradiated at 360 nm, the linker predominantly attains a Z-configuration that significantly stabilizes the helix (Scheme 6.193).1209 Reverse isomerization can proceed either thermally or photochemically at a different wavelength therefore, the process is photochromic (Special Topic 6.15). 

Figure 32 The 124 , 124Z, 124 , and 124Z equilibrium system in which intermolecular host-guest complexation of 1-adamantol competes with cinnamate self-complexation in 124Z and E to Z isomerization about the amide bond occurs while the E stereochemistry about the cinnamate alkene bond is retained. Photoisomerization by irradiating at 300 nm switches the stereochemistry about the cinnamate alkene bond to Z to produce the 125 , 125Z, 125 , and 125Z system in which no competitive cinnamate self-complexation occurs. The cinnamate alkene photoisomerization is reversed by irradiation at 254 nm to complete an on-off photoswitching process.

The selected examples in this chapter demonstrate the success of the development of photoswitchable molecular materials through the rational designs based on the coupling of photochromic moieties into molecules with different functions, such as luminescence, nonlinear optics, liquid crystal, molecular machine, receptor, electrical con-ductor/semiconductor, and many others. It is important to emphasize that photoswitchable materials are not confined only to the selected areas in these examples. With the understanding of both the photoswitching processes and the functional properties at the molecular levels, materials that possess different varieties of photoswitchable functional properties can be readily designed and synthesized. 

Nonadiabatic transitions definitely play crucial roles for molecules to manifest various functions. The theory of nonadiabatic transition is very helpful not only to comprehend the mechanisms, but also to design new molecular functions and enhance their efficiencies. The photochromism that is expected to be applicable to molecular switches and memories is a good example [130]. Photoisomerization of retinal is well known to be a basic mechanism of vision. In these processes, the NT type of nonadiabatic transitions play essential roles. There must be many other similar examples. Utilization of the complete reflection phenomenon can also be another candidate, as discussed in Section V.C. In this section, the following two examples are cosidered (1) photochromism due to photoisomerization between cyclohexadiene (CHD) and hexatriene (HT) as an example of photoswitching molecular functions, and (2) hydrogen transmission through a five-membered carbon ring. 

Chapter 8, by Hobley et al., critically evaluates the existing mechanistic results, especially that of time-resolved investigations, on the photochromic process of spiropyrans and related system. The summary provided should be of great value in the development of these systems as photoswitches. 

Photoswitching of ionic and mechanical processes, iono-photo and mechano-photo switching, will be considered below. 

Nagamura describes in Chapter 9 the fabrication and properties of supramolecular and polymeric systems that display unique linear and nonlinear optical response. Such materials may find application in high-density information processing systems of the future. Chapter 10, by Shinkai and James, describes the properties of supramolecular photoswitchable ion receptors. Finally, in Chapter 11, Ishikawa and Ye discuss the application of state-of-the art fluorescence methods to explore the properties of polymers with nm-scale resolution. 

Like in other chiroptical switches (Section 5.3.1), solvent polarity was found to play an important role. Diastereoselective cyclization was observed in THF and toluene, but not in nonpolar solvents such as n-hexane. Upon photoexcitation, diarylethenes 24 (Scheme 11) can adopt a planar and a twisted conformation, and photocyclization only proceeds through the planar conformation. In the case of chiral diarylethene 27a, there are two diastereomeric planar conformations leading to the diastereomers of the cyclic product 27b. The stereoselectivity in the photocyclization process is enhanced because of a decrease in the excited state energy of the unreactive twisted form, providing a relaxation pathway for the less favorable planar diastereoisomer in more polar solvents. Chiral photochromic diarylethenes are among the most prominent photoswitches known today, featuring nondestructive read-out, excellent reversibility, and the potential for construction of switchable molecular wires and modulation of liquid crystalline phases (see Section 5.5.3).[40,411... 

Redox enzymes are the active component in many electrochemical enzyme electrode biosensor devices.1821 The integration of two different redox enzymes with an electrode support, in which one of the biocatalysts is photoswitchable between ON and OFF states, can establish a composite multisensor array. The biomaterial interface that includes the photoswitchable enzyme in the OFF state electrochemi-cally transduces the sensing event of the substrate corresponding to the nonphoto-switchable enzyme. Photochemical activation of the light-active enzyme leads to the full electrochemical response, corresponding to the analysis of the substrates of the two enzymes. As a result, the processing of the signals transduced by the composite biomaterial interface in the presence of the two substrates permits the assay of the... 

Photoswitchable enzymes could have an important role in controlling biochemical transformations in bioreactors. Various biotechnological processes generate an inhibitor, or alter the environmental conditions (pH, for example) of the reaction medium. Photochemical activation of enzymes that adjust environmental conditions or deplete the inhibitor to a low concentration may maintain the bioreactor at optimal performance. More specifically, integration of the photoswitchable biocataly-tic matrix with a sensory electrode might yield a feedback mechanism in which the sensor element triggers the light-induced activation/deactivation of the photosensitive biocatalyst. 

Photoresponsive systems are seen ubiquitously in nature, and light is intimately associated with the subsequent life processes. In these systems, a photoantenna to capture a photon is neatly combined with a functional group to mediate some subsequent events. Important is the fact that these events are frequently linked with photoinduced structural changes in the photoantennae. This suggests that chemical substances that exhibit photoinduced structural changes may serve as potential candidates for the photoantennae. To date, such photochemical reactions as E/Z isomerism of azobenzenes, dimerization of anthracenes, spiropyran-merocyanine interconversion, and others have been exploited in practical photoantennae. It may be expected that if one of these photoantennae were adroitly combined with a crown ether, it would then be possible to control many crown ether family physical and chemical functions by means of an ON/OFF photoswitch. This is the basic concept underlying the designing of photoresponsive crown ethers. We believe that this is one of the earliest examples of molecular machines . 

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